Re: [Xen-devel] [PATCH v3 16/24] xen/passthrough: Introduce iommu_construct

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Jan,

On 21/01/15 10:48, Jan Beulich wrote:
>>>> On 21.01.15 at 11:37, <julien.grall@xxxxxxxxxx> wrote:
>> On 21/01/2015 10:23, Jan Beulich wrote:
>>>>>> On 20.01.15 at 18:11, <julien.grall@xxxxxxxxxx> wrote:
>>>> While this function is currently only used for DOM0, this will be used
>>>> in a later patch for guest non-PCI passthrough.
>>>
>>> Okay, but you shouldn't break (or alter in [seemingly] benign ways) the
>>> Dom0 case imo.
>>
>> As iommu_hwdom_init is initialized correctly the IOMMU for DOM0, 
>> iommu_construct is a no-op.
>>
>> Would an if ( need_iommu(d) ) will be more clear? Maybe we an assert 
>> (!is_hardware_domain(d)).
> 
> Just think this through properly: iommu_hwdom_init() may leave
> Dom0's ->need_iommu at 0 or 1 (depending on iommu_dom0_strict).
> And iommu_construct() specifically is a nop only when ->need_iommu
> is positive (x86's arch_iommu_populate_page_table() sets it to a
> negative value to indicate "being set up", and I wonder how ARM
> gets away without doing so).

iommu_dom0_strict is always set to 1 when IOMMU is used on ARM (see
check_hwdom_reqs).

Futhermore, we always share the page table with the processor, so we
never need to populate the page table.

Regards,

-- 
Julien Grall

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